The present invention relates to a variable venturi-type carburetor.
The inventor previously proposed a variable venturi-type carburetor, illustrated in FIGS. 1 through 3, in which the upstream end portion b of the tip face of the suction piston a is formed so that it has a V-shaped cross section which expands towards the venturi portion c. A raised wall e is formed on the inner wall of the intake passage d, which faces the upstream end portion b of the tip face of the suction piston a, so that an air-inflow restricting opening f is formed between the raised wall e and the upstream end portion b of the tip face of the suction piston a. In this variable venturi-type carburetor, since the air-inflow restricting opening f has an approximately isosceles triangle shape, as is illustrated in FIG. 2, when the amount of air fed into the cylinder of the engine is small, the lift of the suction piston a is proportional to the amount of air fed into the cylinder of the engine, and, thus, when the amount of air fed into the cylinder of the engine is increased, the suction piston moves, without vibrating, in a direction where the cross-sectional area of the venturi portion c is increased. Consequently, in this variable venturi-type carburetor, even when the engine is accelerated, it is possible to feed fuel from the nozzle g into the intake passage d in an amount which is proportional to the amount of air fed into the cylinder of the engine. On the other hand, this variable venturi-type carburetor is constructed so that the upstream end portion b of the tip face of the suction piston a is completely covered by the raised wall e when the engine is started in order to increase the airflow resistance and thereby produce a great vacuum in the venturi portion c. In addition, this variable venturi-type carburetor is constructed so that the reduced-diameter portion i of the needle h is located within the metering jet j when the engine is started. As mentioned above, when the engine is started, since a great vacuum is produced in the venturi portion c and the reduced-diameter portion i of the needle h is located within the metering jet j, a large amount of fuel can be fed into the intake passage d from the nozzle g.
As will be understood from the above description, in this variable venturi-type carburetor, it is necessary to form the raised wall e so that the upstream end portion b of the tip face of the suction piston a is completely covered by the raised wall e. In order to accomplish this, it is necessary to increase the height of the raised wall e. However, it is impossible to increase the height of the raised wall e very much because when the height of the wall e is increased the airflow resistance becomes great. Therefore, in this variable venturi-type carburetor, the grooves k, into which the tip end of the suction piston a is able to enter, are formed on the inner wall of the intake passage d so that the upstream end portion b of the tip face of the suction piston a is completely covered by the raised wall e. However, a problem occurs in that the formation of such grooves k requires a complicated machining operation.
In order to eliminate such a problem, the inventor later proposed another variable venturi-type carburetor, illustrated in FIGS. 4 and 5, in which the upstream end portion b of the tip face of the suction piston a is formed so that it has a flat surface and in which the tip edge of the raised wall e is formed so that it has a V-shaped cross section. In this variable venturi-type carburetor, since the upstream end portion b of the tip face of the suction piston a has a flat surface, there is an advantage in that it is not necessary to form grooves for receiving the tip end of the suction piston a therein on the inner wall of the intake passage d. However, in this variable venturi-type carburetor, since the carburetor housing is formed by means of a casting operation, it is necessary to machine the raised wall e so that it has a predetermined or V-shaped cross section. In addition, since the raised wall e is located in the interior of the carburetor, a problem occurs in that it is difficult to machine the raised wall e.
It is possible to relatively easily form by means of a machining operation the tip face of the suction piston a so that it has a V-shaped cross section, as is illustrated in FIG. 2, because during machining the tip face of the suction piston a is exposed to the exterior of the carburetor body. Consequently, if the tip face of the suction piston a is formed so that it has a V-shaped cross section, and since it is not necessary to form grooves for receiving the tip end of the suction piston a therein on the inner wall of the intake passage d, manufacture of the carburetor is extremely easy.